This invention relates to particle analyzing devices and more particularly to analyzing devices in which a suspension of particles in a fluid medium is passed through an aperture and the change in impedance of the suspension material when a particle moves through the aperture is detected.
In the Coulter patent, U.S. Pat. No. 2,656,508, a small aperture is provided in a vessel, such as a test tube, and the tube is inserted into another vessel, such as a glass beaker containing a suspension of particles in a fluid medium. Electrodes are disposed in the suspension on opposite sides of the aperture and connected to a current source. Suspension is moved between the vessels through the aperture. As each particle passes through the aperture, it effects a change in the impedance of the suspension between the electrodes to produce a signal or current pulse. When counting blood cells, the suspension is a mixture of blood and a liquid such as a saline solution, in a known ratio, so that by counting the number of signals for a known volume of suspension passing through the aperture, the number of cells for a given quantity of blood is readily ascertainable. The magnitude of the signal determines the size of the particle so that not only can the number of particles be determined but the size or mean size of particle per volume of suspension can be determined. As is well known, such particle analyzing systems are especially useful in determining the number of red and white blood cells per unit volume of blood and the mean cell volume.
Where the particles are small, such as in the case of blood cells, the scanning aperture must, of course, also be small so that a particle moving through the aperture will cause a significant or detectable change in impedance of the suspension across the aperture. Because of the small aperture, foreign particles or debris sometimes present in the fluid medium, such as dust and lint, may cause blockage of the aperture. This clogging of an aperture results in an invalid determination of the number or the size of the particles, as well as a loss of time when an analysis must be repeated. Where the aperture is provided in the wall of a test tube or the like, the tube is disposed in a filled container or beaker, and it is necessary to stop the counting or other analysis and brush or otherwise open the aperture such as by back-flushing the suspension through the aperture. The cleaning problem is further compounded when current densities result in "cooking" the debris onto the aperture side walls. In U.S. Pat. No. 3,395,344, at attempt to overcome the problem of clogged apertures is made by providing a flexible diaphragm with an aperture formed therein which is intended to distort due to increased fluid pressure as debris accumulates at the aperture so as to eventually enlarge the aperture sufficiently to pass the blockage. This is not an entirely satisfactory solution to the problem of clogging since at times there is partial clogging which may vary the effective size of the aperture over an extended period of time before enough debris has accumulated to adequately enlarge the aperture. Also, the aperture varies in size because the diaphragm stretches. Since the effective size of the aperture is generally critical to an accurate analysis, the results from such an apparatus may not always be accurate. In U.S. Pat. No. 3,783,376, a particle scanning or counting cell arrangement is provided by placing a jewel with the aperture therein, in a hole formed in a thin plastic slide member and inserting the slide member between opposed o-rings in the flow path of a receptacle so that the suspension flows through the aperture in the jewel. In this way, the slide may be removed from the scanning cell by sliding it out of the receptacle to remove lodged particles in the aperture. In can also be flicked or vibrated to remove the particles causing the blockage of the aperture while in the receptacle. However, with such a construction, flicking the slide may cause the clogging material to return to the upstream side of the aperture and subsequently clog the aperture or the aperture may only be partially opened by the flicking action. In the above-mentioned devices, it is necessary, in order to be sure that the aperture is completely open, to view the aperture with a microscope.